Light emitting element, light emitting device, and electronic apparatus having first and second composite layers with different metal concentrations

ABSTRACT

It is an object of the present invention to provide a light emitting element with a low driving voltage. In a light emitting element, a first electrode; and a first composite layer, a second composite layer, a light emitting layer, an electron transporting layer, an electron injecting layer, and a second electrode, which are stacked over the first electrode, are included. The first composite layer and the second composite layer each include metal oxide and an organic compound. A concentration of metal oxide in the first composite layer is higher than a concentration of metal oxide in the second composite layer, whereby a light emitting element with a low driving voltage can be obtained. Further, the composite layer is not limited to a two-layer structure. A multi-layer structure can be employed. However, a concentration of metal oxide in the composite layer is gradually higher from the light emitting layer to first electrode side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting element having a layerincluding an organic compound between a pair of electrodes and a lightemitting device using the light emitting element.

2. Description of the Related Art

In recent years, a light emitting device including anelectroluminescence element as a self-light emitting element has beenresearched and developed. In particular, a light emitting device, whichutilizes a so-called an organic electroluminescence element where alayer including an organic compound that emits light by applying anelectric field, has features of a high response speed suitable for anmoving image, low voltage driving, low power consumption driving, andthe like. Therefore, the light emitting device utilizing an organicelectroluminescence has been attracted as a display including a cellphone, a portable information terminal (PDA), and the like of the nextgeneration.

The light emitting element described above is formed with a layerincluding a light emitting substance interposed between a pair ofelectrodes (an anode and a cathode). As emission mechanism thereof, whena voltage is applied between the both electrodes, holes injected fromthe anode and electrons injected from the cathode are recombined witheach other in a luminescent center in a layer including a light emittingsubstance to form molecular exciton. Then, when the molecular excitonreturns to a ground state, the molecular exciton releases energy to emitlight. It is to be noted that singlet excitation and triplet excitationare known as an excitation state. It is considered that light emissioncan be performed through singlet excitation or triplet excitation.

Recently, a driving voltage has been remarkably improved. For example, acomposite layer of a metal oxide such as vanadium pentoxide or dirheniumheptaoxide and an organic compound is to be a hole injecting layer;whereby an energy barrier when injecting holes from an anode to anorganic compound layer can be reduced (see Patent Document 1: JapanesePatent Application Laid-Open No. 2005-123095).

SUMMARY OF THE INVENTION

Incidentally, a light emitting element including an organic substancecan be driven by power consumption potentially lower than liquidcrystal; however, it is yet to be required for various improvements, andfurther low power consumption is desired.

The light emitting element described in Patent Document 1 has a holetransporting layer made only of an organic compound. Although a materialused in such a hole transporting layer has conductivity to some extent,resistance thereof can hardly be low.

Thus, it is an object of the present invention to provide a lightemitting element and a light emitting device with further low powerconsumption.

In order to achieve the above object, a means below is implemented inthe present invention.

A light emitting element has a pair of electrodes and a plurality oflayers interposed between the pair of electrodes. The plurality oflayers includes at least a light emitting layer and a composite layerincluding metal oxide and an organic compound between one of the pair ofthe electrodes and the light emitting layer.

The composite layer has a structure in which a first region and a secondregion are alternately stacked. A concentration of metal oxide in thefirst region is equal to or higher than a concentration of metal oxidein the second region, and the highest concentration of metal oxide inthe first region is the same or at most eight times as the lowestconcentration of metal oxide in the second region.

Each concentration of metal oxide in the composite layer changeperiodically in the stacked direction. One cycle of a periodic change is12 nm or less.

According to one mode of a light emitting element of the presentinvention, a first electrode, and a first composite layer, a secondcomposite layer, a light emitting layer, an electron transporting layer,and a second electrode which are sequentially stacked over the firstelectrode, are included; the first composite layer and the secondcomposite layer each include metal oxide and an organic compound; and anaverage concentration of metal oxide in the first composite layer ishigher than an average concentration of metal oxide in the secondcomposite layer.

In accordance with the above invention, each of the first compositelayer and the second composite layer has a structure in which a firstregion and a second region are alternately stacked. A concentration ofmetal oxide in the first region is equal to or higher than aconcentration of metal oxide in the second region, and the highestconcentration of metal oxide in the first region is the same or at mosteight times as the lowest concentration of metal oxide in the secondregion.

In accordance with the above invention, each concentration of metaloxide in the first composite layer and the second composite layer changeperiodically in the stacked direction. One cycle of a periodic change is12 nm or less.

According to another mode of a light emitting element of the presentinvention, a first electrode, and a first composite layer, a secondcomposite layer, a third composite layer, a light emitting layer, anelectron transporting layer, and a second electrode which aresequentially stacked over the first electrode, are included; the firstcomposite layer, the second composite layer, and the third compositelayer each include metal oxide and an organic compound; an averageconcentration of metal oxide in the first composite layer is higher thanan average concentration of metal oxide in the second composite layer;and the average concentration of metal oxide in the second compositelayer is higher than an average concentration of metal oxide in thethird composite layer.

In accordance with the above invention, each of the first compositelayer, the second composite layer, and the third composite layer has astructure in which a first region and a second region are alternatelystacked. A concentration of metal oxide in the first region is equal toor higher than a concentration of metal oxide in the second region. Thehighest concentration of metal oxide in the first region is the same orat most eight times as the lowest concentration of metal oxide in thesecond region,

In according with the above invention, each concentration of metal oxidein the first composite layer, the second composite layer, and the thirdcomposite layer change periodically in the stacked direction. One cycleof a periodic change is 12 nm or less.

In accordance with the above invention, an average concentration ofmetal oxide refers to a concentration of metal oxide in the entirecomposite layer. The composite layer has a structure in which tworegions having different concentrations of metal oxide from each other,in other words, a first region and a second region, are alternatelystacked.

According to another mode of a light emitting element of the presentinvention, a first electrode, and a composite layer, a light emittinglayer, an electron transporting layer, and a second electrode, which aresequentially stacked over the first electrode, are included; thecomposite layer includes metal oxide and an organic compound; and themetal oxide in the composite layer has a concentration gradient from thefirst electrode to the light emitting layer side.

In accordance with the above invention, the concentration of metal oxidein the composite layer has the lowest concentration in a surface incontact with the light emitting layer.

In accordance with the above invention, the concentration of metal oxidein the composite layer is 0 wt % or more and 3 wt % or less in a surfacein contact with the light emitting layer.

In accordance with the above invention, the metal oxide is one or pluralkinds of titanium oxide, vanadium oxide, chromium oxide, zirconiumoxide, niobium oxide, molybdenum oxide, hafnium oxide, tantalum oxide,tungsten oxide, and rhenium oxide.

In accordance with the above invention, the organic compound has a holetransporting property.

In accordance with the above invention, the organic compound has anarylamine skeleton or a carbazole skeleton.

In accordance with the present invention, it is possible to provide alight emitting element where a hole transporting property is improved,and then, a driving voltage is reduced. In addition, by manufacturing alight emitting device using the light emitting element, a light emittingdevice having high reliability, low power consumption, and longlifetime, and an electronic apparatus provided with the light emittingdevice can be provided.

In particular, by shortening one cycle of periodic change of aconcentration of metal oxide in a composite layer in the stackeddirection, an element superior in a current characteristic can bemanufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are views for explaining a light emitting element of thepresent invention;

FIG. 2 is a view for explaining a light emitting element of the presentinvention;

FIGS. 3A to 3C are views for explaining a light emitting element of thepresent invention;

FIG. 4 is a view for explaining a method for manufacturing a lightemitting element of the present invention;

FIG. 5 is a view for explaining a method for manufacturing a lightemitting element of the present invention;

FIG. 6 is a view for explaining a method for manufacturing a lightemitting element of the present invention;

FIG. 7 is a view for explaining a method for manufacturing a lightemitting element of the present invention;

FIGS. 8A to 8C are views for explaining a method for manufacturing alight emitting element of the present invention;

FIGS. 9A and 9B are views for explaining a light emitting device of thepresent invention;

FIGS. 10A to 10D are views for explaining an electronic apparatus usinga light emitting element of the present invention;

FIG. 11 is a photograph for showing an observation result of a compositelayer by a transmission electron microscope;

FIG. 12 is a photograph for showing an observation result of a compositelayer by a transmission electron microscope;

FIG. 13 is a graph for showing a current density-voltage characteristicin a composite layer; and

FIG. 14 is a schematic view of an evaporation device in a case ofexamining a concentration of metal oxide in a composite layer.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be described below withreference to drawings. However, the present invention is not limited tothe following description, and it is to be easily understood thatvarious changes and modifications will be apparent to those skilled inthe art, unless such changes and modifications depart from the contentand the scope of the invention. Therefore, the present invention is notconstrued as being limited to the description of the followingembodiment modes. It is to be noted that the same portion may be denotedby the same reference numeral in differing drawings in a structure ofthe present invention described below.

(Embodiment Mode 1)

A mode of a light emitting element of the present invention will beexplained with reference to FIGS. 1A to 1C. In this mode, a lightemitting element is constituted by a first electrode 102; a firstcomposite layer 103 a, a second composite layer 103 b, a light emittinglayer 104, an electron transporting layer 105, and an electron injectinglayer 106, which are sequentially stacked over the first electrode 102;and a second electrode 107 formed thereover. It is to be noted that thepresent embodiment mode is explained, in which the light emittingelement is formed over a substrate 101, and the first electrode 102 andthe second electrode 107 respectively serve as an anode and a cathode.

As a material used for the substrate 101, for example, a quartzsubstrate, a glass substrate, a plastic substrate, a flexible substrate,or the like can be used. Other materials may be used as far as theyserve as a support in a manufacturing process of the light emittingelement.

An anode material for forming the first electrode 102 is notparticularly limited, and a metal, an alloy, or an electricallyconductive compound each of which has a high work function (workfunction of 4.0 eV or more), a mixture thereof, or the like ispreferable. As a specific example of such anode materials, the followingcan be used: ITO (indium tin oxide), ITO containing silicon oxide, IZO(indium zinc oxide) formed using a target in which zinc oxide (ZnO) of 2to 20 wt % is mixed into indium oxide, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), nitride of a metal material such asTiN, and the like.

The first composite layer 103 a is a layer including metal oxide and anorganic compound. As metal oxide used for the first composite layer 103a, a transition metal oxide is preferable, specifically, titanium oxide,zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalumoxide, chromium oxide, molybdenum oxide, tungsten oxide, manganeseoxide, rhenium oxide, or the like can be used. In particular, vanadiumoxide, molybdenum oxide, tungsten oxide, and rhenium oxide arepreferable because of a high electron accepting property. Above all,molybdenum oxide is preferable because of stability even under anatmosphere and easiness of treatment.

Further, as an organic compound used for the first composite layer 103a, a material superior in a hole transporting property is preferable. Inparticular, an organic material having an arylamine skeleton ispreferable. For example, a compound of aromatic amines (namely, having abenzene ring-nitrogen bond) such as4,4′-bis(N-{4-[N,N′-bis(3-methylphenyl)amino]phenyl}-N-phenylamino)biphenyl(abbreviated as DNTPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviatedas DPAB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviated as TPD), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviated as a-NPD),4,4′-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl(abbreviated as DFLDPBi),4,4′-bis[N-(4-biphenylyl)-N-phenylamino]biphenyl (abbreviated as BBPB),1,5-bis(diphenylamino)naphthalene (abbreviated as DPAN),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviated as TDATA),and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviated as MTDATA) can be used. Alternatively, an organic materialhaving a carbazole skeleton is preferably used. For example, a compoundof N-(2-naphthyl)carbazole (abbreviated as NCz),4,4′-di(N-carbazolyl)biphenyl (abbreviated as CBP),9,10-bis[4-(N-carbazolyl)phenyl]anthracene (abbreviated as BCPA),3,5-bis[4-(N-carbazolyl)phenyl]biphenyl (abbreviated as BCPBi), or1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviated as TCPB) can beused. In addition, aromatic hydrocarbon such as anthracene or9,10-diphenylanthracene (DPA) or aromatic hydrocarbon containing atleast one vinyl skeleton such as 4,4′-bis(2,2-diphenylvinyl)biphenyl(DPVBi) may be used. It is to be noted that other materials may be usedas far as they are substances having a higher hole transporting propertythan an electron transporting property.

The second composite layer 103 b is also a layer including metal oxideand an organic compound as the same as the first composite layer 103 b.As metal oxide and an organic compound used for the second compositelayer 103 b, the same material as the above first composite layer 103 acan be used. It is to be noted that a concentration of metal oxide inthe second composite layer 103 b is to be lower than a concentration ofmetal oxide in the first composite layer 103 a. However, as aconcentration of metal oxide in the second composite layer 103 b, aconcentration is needed to be selected, which does not quench lightemitted by adjacency of the light emitting layer 104 and the metal oxideor quenches only a small part thereof. It is to be noted that mostfavorable concentration of metal oxide in the composite layer at asurface in contact with the light emitting layer is more than 0 wt % and3 wt % or less. However, the concentration of metal oxide is notparticularly limited as far as the above condition is fulfilled. Aconcentration percentage by weight [wt %] can be found by a numericalformula 1.Concentration of metal oxide[wt %]=(weight of metal oxide)/(weight ofcomposite layer)×100  [Numerical Formula 1]

Further, in order to lower the concentration of metal oxide in thecomposite layer at a surface in contact with the light emitting layer, aconcentration of metal oxide contained in the second composite 103 b mayhave a concentration gradient in a direction of a film thickness. It isto be noted that materials used as an organic compound for the firstcomposite layer 103 a and the second composite layer 103 b may be thesame or different with each other.

The first composite layer 103 a and the second composite layer 103 b maybe formed using the above material by co-evaporation using resistanceheating, co-evaporation using resistance heating evaporation andelectron beam evaporation (EB evaporation), simultaneous deposition bysputtering and resistance heating, or the like. Further, the firstcomposite layer 103 a and the second composite layer 103 b may be formedby a wet method such as a sol-gel method.

The light emitting layer 104 is a light emitting layer including asubstance having high luminosity. The light emitting layer is notparticularly limited. However, a layer serving as a light emitting layermainly has two types: one is a host-gate type layer in which a lightemitting substance is dispersed in a material (host material) having alarger energy gap than that of a substance (light-emitting substance orguest material), which becomes a light emission center; the other is alayer forming a light emitting layer using only a light emittingmaterial. The former layer in which concentration quenching hardlyoccurs is preferable. As a light emitting substance,4-dicyanomethylene-2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(abbreviated as DCJT),4-dicyanomethylene-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran,periflanthene,2,5-dicyano-1,4-bis(10-methoxy-1,1,7,7-tetramethyljulolidyl-9-enyl)benzene,N,N-dimethylquinacridone (abbreviated as DMQd), coumarin 6, coumarin545T, tris(8-quinolinolato)aluminum (abbreviated as Alq₃),9,9-bianthryl, 9,10-diphenylanthracene (abbreviated as DPA),9,10-bis(2-naphthyl)anthracene (abbreviated as: DNA),2,5,8,11-tetra-t-butylperylene (abbreviated as TBP), or the like can begiven. As a host material, an anthracene derivative such as9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation t-BuDNA); acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl (abbreviatedas CBP); a metal complex such as tris(4-methyl-8-quinolinolato) aluminum(abbreviated as Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviated as BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated asBAlq), bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated as Znpp₂),and bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviated as ZnBOX); orthe like can be used. As a material for forming a light emitting layerusing only a light emitting substance, tris(8-quinolinolato)aluminum(abbreviated as Alq₃), 9,10-bis(2-naphthyl)anthracene (abbreviated asDNA), and bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviated as BAlq), or the like can be given.

The electron transporting layer 105 is preferably formed using amaterial that can transport electrons, which is injected into a layercontaining a light emitting substance from an electrode serving as acathode side, toward the light emitting layer. As a specific example ofsuch a material, a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum(abbreviated as Alq₃), tris(8-quinolinolato)gallium (abbreviated asGaq₃), tris(4-methyl-8-quinolinolato)aluminum (abbreviated as Almq₃), orbis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviated as BeBq₂) canbe given. In addition, a metal complex having an oxazole based orthiazole based ligand, such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviated as Zn(BOX)₂), orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviated as Zn(BTZ)₂),or the like can be used as a material for forming the electrontransporting layer 105. Further,2-(4-biphenylyl)-5-(4-tert-buthylphenyl)-1,3,4-oxadiazole (abbreviatedas PBD), 1,3-bis[5-(p-tert-buthylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated as OXD-7),3-(4-tert-buthylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated as TAZ),3-(4-tert-buthylphenyl)-4-(4-ethylpheyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated as p-EtTAZ), bathophenanthroline (abbreviated as BPhen),bathocuproin (abbreviated as BCP), and an inorganic material such astitanium oxide may be used.

The electron injecting layer 106 is a layer having a function forsupporting electron injection from an electrode serving as a cathodeinto the electron transporting layer 105. The electron injecting layer106 is not particularly limited, and a layer formed using an alkalimetal or alkaline earth metal compound such as lithium fluoride (LiF),cesium fluoride (CsF), or calcium fluoride (CaF2) can be used. Besides,the electron injecting layer 106 may be formed using a mixed layer ofany one of the above electron transporting materials and a substanceshowing an electron donating property with respect to the electrontransporting material. As a substance showing an electron donatingproperty, for example, a metal having a low work function can be given.Specifically, an alkaline metal and an alkaline earth metal arepreferable, in particular, Li, Mg, and Cs are preferable. Further, analkaline metal complex such as lithium acetylacetonate (abbreviated asLi(acac)) and 8-quinolinolato-lithium (abbreviated as Liq) can also beeffectively used.

As a substance for forming the second electrode 107, a metal, an alloy,or an electrically conductive compound each of which has a low workfunction (work function of 3.8 eV or less), a mixture thereof, or thelike can be used. As a specific example of such cathode materials, thefollowing can be used: an element belonging to Group 1 or 2 in theperiodic table, that is, an alkali metal such as lithium (Li) or cesium(Cs) or an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr). In addition, by using a material superior in a functionof injecting electrons particularly in the electron injecting layer 106,various conductive materials including the above described materials forthe first electrode 102 such as Al, Ag, ITO and ITO containing siliconoxide can be used for the second electrode 107, regardless the level ofthe work function. Further, without limiting to the electron injectinglayer 106, a layer superior in a function of injecting electrons isstacked with the second electrode 107 on the light emitting layer 104side of the second electrode 107 to obtain the same effect.

It is to be noted that the first electrode 102 and the second electrode107 are formed by respectively depositing the above anode material andcathode material by an evaporation method, a sputtering method, or thelike. Further, the first electrode 102 and the second electrode 107 maybe formed using a droplet including a conductor by an inkjet method orthe like. As a method for forming the light emitting layer 104, theelectron transporting layer 105, and the electron injecting layer 106,in addition to an evaporation method, an inkjet method, a spin coatingmethod, or the like can be used. Different forming methods may be usedin each electrode and each layer.

A voltage is applied to a light emitting element of the presentinvention having the above structure so that potential of the firstelectrode 102 gets higher than potential of the second electrode 107,whereby the light emitting element can be made to emit light.

Light emission is extracted outside through one or both of the firstelectrode 102 and the second electrode 107. Accordingly, one or both ofthe first electrode 102 and the second electrode 107 are manufacturedusing a light-transmitting substance. In a case where only the firstelectrode 102 is made of a light-transmitting substance, light emissionis extracted from a substrate 101 side through the first electrode 102as shown in FIG. 1A. In addition, in a case where only the secondelectrode 107 is made of a light-transmitting substance, light emissionis extracted from the opposite side to the substrate 101 through thesecond electrode 107 as shown in FIG. 1B. In a case where both the firstelectrode 102 and the second electrode 107 are made of alight-transmitting substance, light emission is extracted from both ofthe substrate 101 side and the opposite side to the substrate 101through the first electrode 102 and the second electrode 107 as shown inFIG. 1C.

In the first composite layer 103 a and the second composite layer 103 bincluding metal oxide and an organic compound, a driving voltage is notincreased even when a film thickness is increased. Therefore, an opticaldesign utilizing a micro cavity effect and an interference effect oflight can be performed by adjusting a thickness of the first compositelayer 103 a and the second composite layer 103 b. Thus, a highly qualitylight emitting element that has superiority in color purity and lesscolor change that depends on a view angle can be manufactured. Further,a film thickness can be selected, which prevents the first electrode 102and the second electrode 107 from being short-circuited, which is causedby unevenness generated over a surface of the first electrode 102 informing the electrode and a fine residue remaining over an electrodesurface.

Since the first composite layer 103 a in contact with the firstelectrode 102 and the second composite layer 103 b have highcarrier-density, the first composite layer 103 a and the secondcomposite layer 103 b have an excellent hole transporting property.Thus, a driving voltage can be reduced. Further, the first electrode 102and the first composite layer 103 a can be made extremely closer to anohmic contact. Therefore, a range of selection of an electrode materialis extended.

The first composite layer 103 a and the second composite layer 103 bused in the present invention can be formed by vacuum evaporation. Thus,in a case where another layer is formed by vacuum evaporation, any layercan be formed in the same vacuum apparatus. Accordingly, it is possibleto manufacture the light emitting element without being exposed to anatmosphere before the light emitting element is completely sealed;therefore, the process becomes easy, and a yield can be improved.

Since the first composite layer 103 a and the second composite layer 103b include an organic material and an inorganic material, stressgenerated between the electrode and the light emitting layer can berelieved.

It is found that, as a concentration of metal oxide contained in thecomposite layer is higher, a refraction index has a tendency to increasemoderately. In the light emitting element of the present invention, aconcentration of metal oxide contained in the first composite layer 103a is higher than that of the second composite layer 103 b as describedabove. Therefore, by approximately selecting a concentration of metaloxide, the first composite layer 103 a can have a larger refractionindex than the second composite layer 103 b. Though the first electrode102 has generally a large refraction index compared to an organiccompound used for a light emitting element, by stacking composite layershaving different concentrations of metal oxide with each other, eachrefraction index of the second composite layer 103 b, the firstcomposite layer 103 a, and the first electrode 102 can be sequentiallyenlarged.

In accordance with a relation of such a refraction index, in a casewhere light generated in the light emitting layer 104 is extracted fromthe first electrode 102 side, namely, in a case where the firstelectrode 102 is a transparent electrode, a difference of a refractionindex in each interface (for example, an interface between the secondcomposite layer 103 b and the first composite layer 103 a, and aninterface between the first composite layer 103 a and the firstelectrode 102) can be reduced. Since reflectivity of light is reduced inan interface of substances, each of which refraction index is furtherequivalent, light generated in the light emitting layer 104 can beefficiently extracted from the light emitting element. As describedabove, the efficiency of the extracting light is improved; therefore, along lifetime light emitting element with low power consumption can beobtained.

Further, by using different organic compounds in the first compositelayer 103 a and the second composite layer 103 b from each other, thefirst composite layer 103 a and the second composite layer 103 b, whichcan obtain the above relation of the refraction index, can bemanufactured with a small amount of metal oxide while fulfilling acondition where a concentration of metal oxide in the first compositelayer 103 a is higher than that of the second composite layer 103 b.

In order to extract light efficiently from the light emitting element,an unevenness process may be implemented on a surface of the firstelectrode 102. In this case, it is preferable that the first compositelayer 103 a be formed by using a wet method such as a spin coatingmethod to be a film having highly planarity, in consideration of a filmstacked thereover.

It is to be noted that another structure other than the above structuremay be employed as far as the structure includes the first compositelayer 103 a, the second composite layer 103 b, and the light emittinglayer 104. In the present embodiment mode, a structure in which thefirst electrode 102, the first composite layer 103 a, the secondcomposite layer 103 b, the light emitting layer 104, the electrontransporting layer 105, the electron injecting layer 106, and the secondelectrode 107 are sequentially stacked over the substrate 101, isprovided. However, a structure, in which the first substrate 102 to thesecond substrate 107 are sequentially stacked over the substrate 101 inreverse order to the above structure, may be employed.

In such a manner, the stacked-layer structure is not particularlylimited. A layer made of a substance having a high electron transportingproperty, a substance having a high electron injecting property, asubstance having a high hole injecting property, a bipolar (a substancehaving a high electron and hole transporting property) substance, or thelike may be freely combined with a stacked layer of the light emittinglayer, the first composite layer, and the second composite layer to forma stacked structure.

Further, the composite layer is not limited to a two-layer of the firstcomposite layer 103 a and the second composite layer 103 b, and acomposite layer of three-layer of a first composite layer 203 a, asecond composite layer 203 b, and a third composite layer 203 c may beemployed as shown in FIG. 2. However, each concentration of metal oxidecontained in a first electrode 102, the first composite layer 203 a, anda second composite layer 203 b, and the third composite layer 203 c aresequentially lowered.

Furthermore, the composite layer is not limited to a two-layer structureand a three-layer structure. When the number of stacked layers of thecomposite layer is “n”, n>1 may be fulfilled. However, a concentrationof metal oxide contained in the composite layers becomes lower from afirst electrode 102 to a light emitting layer 104 side. It is to benoted that, as a concentration of metal oxide included in the compositelayer in contact with the light emitting layer, a concentration isneeded to be selected, which does not quench light emitted by adjacencyof the light emitting layer and the metal oxide or quenches only a smallpart thereof. Further, a composite layer in contact with the lightemitting layer may have a concentration gradient from the firstelectrode toward the light emitting layer side. In Embodiment Mode 2, acase where the number of stacked layers of a composite layer “n” isinnumerability, in other words, metal oxide in a composite layer has aconcentration gradient, will be shown.

(Embodiment Mode 2)

One mode of a light emitting element of the present invention will beexplained with reference to FIGS. 3A to 3C. The same portion withEmbodiment Mode 1 is denoted by the same reference numeral, and detailedexplanation will be omitted.

A light emitting element is formed of a first electrode 102; a compositelayer 303, a light emitting layer 104, an electron transporting layer105, and an electron injecting layer 106, which are sequentially stackedover the first electrode 102; and a second electrode 107 providedthereover. It is to be noted that the present embodiment mode isexplained, in which the first electrode 102 and the second electrode 107respectively serves as an anode and a cathode.

The first electrode 102 is formed over a substrate 101. Further, thecomposite layer 303 is formed over the first electrode 102. Thecomposite layer 303 is a layer including metal oxide and an organiccompound. As metal oxide used for the composite layer 303, a transitionmetal oxide is preferable, specifically, titanium oxide, zirconiumoxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide,rhenium oxide, or the like can be used. In particular, vanadium oxide,molybdenum oxide, tungsten oxide, or rhenium oxide are preferablebecause of a high electron accepting property. Above all, molybdenumoxide is preferable because of stability even under an atmosphere andeasiness of treatment.

Further, as an organic compound used for the composite layer 303, amaterial superior in a hole transporting property is preferable. Inparticular, an organic material having an arylamine skeleton ispreferable. For example, a compound of aromatic amines (namely, having abenzene ring-nitrogen bond) such as4,4′-bis(N-{4-[N,N′-bis(3-methylphenyl)amino]phenyl}-N-phenylamino)biphenyl(abbreviated as DNTPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviatedas DPAB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviated as TPD), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviated as a-NPD),4,4′-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl(abbreviated as DFLDPBi),4,4′-bis[N-(4-biphenylyl)-N-phenylamino]biphenyl (abbreviated as BBPB),1,5-bis(diphenylamino)naphthalene (abbreviated as DPAN),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviated as TDATA),and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviated as MTDATA) can be used. Alternatively, an organic materialhaving a carbazole skeleton is preferably used. For example, a compoundof N-(2-naphthyl)carbazole (abbreviated as NCz),4,4′-di(N-carbazolyl)biphenyl (abbreviated as CBP),9,10-bis[4-(N-carbazolyl)phenyl]anthracene (abbreviated as BCPA),3,5-bis[4-(N-carbazolyl)phenyl]biphenyl (abbreviated as BCPBi), or1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviated as TCPB) can beused. It is to be noted that other materials may be used as far as theyare substances having a higher hole transporting property than anelectron transporting property.

The composite layer 303 formed of the above material has a structure inwhich a concentration of metal oxide contained in the composite layer isgradually lowered from the first electrode side 102 toward the lightemitting layer 104 side. Also, the composite layer 303 is a layer havinga concentration gradient in the film thickness direction. It is to benoted that a concentration of metal oxide in the composite layer 303 ata surface in contact with the light emitting layer 104 is preferablymore than 0 wt % or more and 3 wt % or less.

The composite layer 303 may be formed using the above material byco-evaporation using resistance heating, co-evaporation using resistanceheating evaporation and electron beam evaporation (EB evaporation),simultaneous deposition by sputtering and resistance heating, or thelike. However, amount of evaporation of metal oxide is needed to bereduced with time.

Next, the light emitting layer 104, the electron transporting layer 105,the electron injecting layer 106, and the second electrode 107 areformed over the composite layer 303. A material and a manufacturingmethod of the electrode and each layer are the same as those inEmbodiment Mode 1.

A voltage is applied to a light emitting element of the presentinvention, which has the above structure, so that potential of the firstelectrode 102 gets higher than potential of the second electrode 107,whereby the light emitting element can be made to emit light.

Light emission is extracted outside through one or both of the firstelectrode 102 and the second electrode 107. Accordingly, one or both ofthe first electrode 102 and the second electrode 107 are manufacturedusing a light-transmitting substance. In a case where only the firstelectrode 102 is made of a light-transmitting substance, light emissionis extracted from a substrate 101 side through the first electrode 102as shown in FIG. 3A. In addition, in a case where only the secondelectrode 107 is made of a light-transmitting substance, light emissionis extracted from the opposite side to the substrate 101 through thesecond electrode 107 as shown in FIG. 3B. In a case where both the firstelectrode 102 and the second electrode 107 are made of alight-transmitting substance, light emission is extracted from both ofthe substrate 101 side and the opposite side to the substrate 101through the first electrode 102 and the second electrode 107 as shown inFIG. 3C.

In the composite layer 303 including metal oxide and an organiccompound, a driving voltage is not increased even when a film thicknessis increased. Therefore, an optical design utilizing a micro cavityeffect and an interference effect of light can be performed by adjustinga thickness of the composite layer 303. Thus, a highly quality lightemitting element that has superiority in color purity and less colorchange that depends on a view angle can be manufactured. Further, a filmthickness can be selected, which prevents the first electrode 102 andthe second electrode 107 from being short-circuited, which is caused byunevenness generated over a surface of the first electrode 102 and afine residue remaining over an electrode surface.

The composite layer 303 has highly carrier-density, the composite layer303 has an excellent hole transporting property. Thus, a driving voltagecan be reduced. Further, the electrode and the composite layers can bemade extremely closer to an ohmic contact. Therefore, a range ofselection of an electrode material is extended.

The composite layer 303 used in the present invention can be formed byvacuum evaporation. Thus, in a case where another layer is formed byvacuum evaporation, any layer can be formed in the same vacuumapparatus. Accordingly, it is possible to manufacture a light emittinglayer without being exposed in an atmosphere before the light emittingelement is completely sealed; therefore, the process becomes easy, and ayield can be improved.

Since the composite layer 303 includes an organic material and aninorganic material, stress generated between the electrode and the lightemitting layer can be relieved.

In the present embodiment mode, metal oxide in the composite layer 303has a concentration gradient, and the concentration becomes lowergradually from the first electrode 102 to the light emitting layer 104side. Therefore, a refraction index of the composite layer 303 can becontinuously changed from the light emitting layer 104 to firstelectrode 102 side. In addition, a concentration of metal oxide in thecomposite layer 303 at a surface in contact with the first electrode 102is appropriately selected so as to be close to a refraction index of thefirst electrode 102, whereby a difference of a refraction index can bereduced also at an interface between the composite layer 303 and thefirst electrode 102. Therefore, when light emitted generated in thelight emitting layer 104 is extracted from the first electrode 102 side,a difference of a refraction index can be reduced in the composite layer303 and at an interface between the composite layer 303 and the firstelectrode 102. Since reflectivity of light is reduced in an interface ofsubstances, each of which refraction index is further equivalent, lightgenerated in the light emitting layer 104 can be efficiently extractedfrom the light emitting element. As described above, the efficiency ofthe extracting light is improved; therefore, a light emitting elementwith a long lifetime and low power consumption can be obtained.

In order to extract light efficiently from the light emitting element,an unevenness process may be implemented on a surface of the firstelectrode 102. In this case, it is preferable that the composite layer303 be formed to be a film having highly planarity in consideration of afilm stacked thereover.

It is to be noted that another structure other than the above structuremay be employed as far as the structure includes the composite layer 303and the light emitting layer 104. In the present embodiment mode, astructure in which the first electrode 102, the composite layer 303, thelight emitting layer 104, the electron transporting layer 105, theelectron injecting layer 106, and the second electrode 107 aresequentially stacked over the substrate 101, is provided. However, astructure, in which the first electrode 102 to the second electrode 107are sequentially stacked over the substrate 101 in reverse order to theabove structure, may be employed.

That is, the stacked-layer structure of is not particularly limited.Layers made of a substance having a high electron transporting property,a substance having a high electron injecting property, a substancehaving a high hole injecting property, a bipolar (a substance having ahigh electron and hole transporting property) substance, and the likemay be freely combined with a stacked-layer of the light emitting layer,the first composite layer, and the second composite layer to form astacked structure.

The present embodiment mode can be freely combined with the structure ofEmbodiment Mode 1.

(Embodiment Mode 3)

An evaporation apparatus used for implementation of the presentinvention and a method for forming a composite layer with the use of theevaporation apparatus by co-evaporation will be explained with referenceto FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIGS. 8A to 8C.

In an evaporation apparatus used for implementation of the presentinvention, a transfer chamber 402 as well as a treatment chamber 401where evaporation is performed with respect to an object is provided asshown in FIG. 4. The object is transferred to the treatment chamber 401through the transfer chamber 402. It is to be noted that the transferchamber 402 is provided with an arm 403 for transferring the object.

In the treatment chamber 401 as shown in FIG. 5, a holder for holdingthe object, an evaporation source 501 a in which metal oxide is held,and an evaporation source 510 b in which an organic compound is held areprovided. In FIG. 5, the holder for holding the object is constituted bya first rotating plate 502 that is rotated around an axis 503 and aplurality of second rotating plates 504 a to 504 d provided over thefirst rotating plate 502. The second rotating plates 504 a to 504 d maybe independently rotated around each axis that is provided in each ofthe second rotating plates 504 a to 504 d, separately from the axis 503.Objects 505 a to 505 d are respectively held over the second rotatingplates 504 a to 504 d.

In FIG. 5, the object 505 a is held over the second rotating plate 504a, the object 505 b is held over the second rotating plate 504 b, theobject 505 c is held over the second rotating plate 504 c, and theobject 505 d is held over the second rotating plate 504 d. Though it isnot shown here, sliding shutters are provided over each evaporationsource.

A composite layer is formed by heating each material held in theevaporation sources 501 a and 501 b and evaporating them.

A composite layer made of molybdenum oxide and DNTPD is formed under acondition where each distance between an upper end of the evaporationsources 501 a and 501 b and the objects 505 a to 505 d is 270 mm, anevaporation rate of the organic compound is 0.4 nm/s, and the number ofrotations of the first rotating plate 502 is 2 rpm. A cross section ofthe obtained subject 505 a, in other words, a cross section of thecomposite layer is observed with the use of a transmission electronmicroscope (TEM). The obtained TEM photograph is shown in FIG. 11. Aweight ratio of the molybdenum oxide and DNTPD in the composite layer isset to be 0.67:1.

From FIG. 11, it can be seen that a first region with a dark color ((a)in FIG. 11) and a second region with a light color ((b) in FIG. 11) arealternately provided. The first region with a dark color is a regionwhere the average atomic weight is high, while the second region with alight color is a region where the average atomic weight is low. Sincethe average atomic weight of the metal oxide is higher than that of theorganic compound in the layer containing a composite material of thepresent invention, the first region with a dark color in the TEMphotograph corresponds to a region containing a larger amount of metaloxide, while the second region with a light color corresponds to aregion containing a larger amount of an organic compound. Accordingly,in FIG. 11, the first region with a dark color is a region having a highconcentration of molybdenum oxide, and the second region with a lightcolor is a region having a high concentration of DNTPD.

The first region having a high concentration of metal oxide is formedunder a condition where a distance between the object 505 a and theevaporation source 501 a in which metal oxide is held is shorter than adistance between the object 505 a and the evaporation source 501 b inwhich an organic compound is held. Alternatively, the second regionhaving a low concentration of metal oxide is formed under a conditionwhere a distance between the object 505 a and the evaporation source 501b is shorter than a distance between the object 505 a and theevaporation source 501 a. Accordingly, it is found that a concentrationof metal oxide in the composite layer has a difference depending on anevaporation position.

Therefore, in order to examine a difference of concentration of metaloxide due to the evaporation position, each concentration of metal oxidein a position A that is closest to the evaporation source 501 a and aposition B that is closest to the evaporation source 501 b is examined.A schematic view of the evaporation apparatus used for the aboveexamination is shown in FIG. 14.

Molybdenum trioxide for forming molybdenum oxide that is metal oxide,and DNTPD that is an organic compound are respectively held in theevaporation source 501 a and the evaporation source 501 b. The metaloxide and the organic compound are each evaporated under the samecondition, and then, each concentration of metal oxide in the compositelayer, which is to be formed in the positions A and B, is estimated fromeach film thickness. A result thereof is shown in Table 1.

TABLE 1 a film [nm] A concentration of molybdenum oxide Molybdenum inthe composite layer Position oxide DNTPD [vol %] (calculated value) A 6560 52.0 B 15 220 6.38

As shown in Table 1, when only the evaporation source 501 a isevaporated in the position A, a molybdenum oxide film with a thicknessof 65 nm is formed on the object. Alternatively, when only theevaporation source 501 b is evaporated under the same condition, a DNTPDfilm with a thickness of 60 nm is formed. Accordingly, in a case wherethe evaporation sources 501 a and 501 b are concurrently evaporated, inother words, co-evaporated, a concentration of molybdenum oxide in thecomposite layer formed in the position A is calculated to be 52.0 [vol%]. A concentration by volume percent [vol %] display can be found by anumerical formula 2.Concentration of metal oxide[vol %]=(volume of molybdenum oxide)/(volumeof molybdenum oxide+volume of DNTPD)×100  [Numerical Formula 2]

On the other hand, when only the evaporation source 501 a is evaporatedin the position B, a molybdenum oxide film with a thickness of 15 nm isformed on the object. Alternatively, when only the evaporation source501 b is evaporated under the same condition, a DNTP film with athickness of 220 nm is formed. Accordingly, in a case where theevaporation sources 501 a and 501 b are evaporated, in other words,co-evaporated, a concentration of molybdenum oxide in the compositelayer formed in the position B is calculated to be 6.38 [vol %].

Therefore, the high concentration region of molybdenum oxide is found tohave 8.15 times as molybdenum oxide as the low concentration region.

Though the above example is an extreme case, such a phenomenon may becaused in a case where a rotation speed of the first rotating plate 502is small or in a case where an evaporation rate is extremely high. Acomposite layer used for a light emitting element is preferably uniform.In the composite layer, a high concentration region of metal oxide isdesired to be at least the same or at most eight times as a lowconcentration region. A rotation speed, an evaporation rate, a distancebetween the object and the evaporation source, a distance of theevaporation sources, a distance between the second rotating plate andthe axis of the first rotating plate, and the like are appropriatelydesigned to perform evaporation so as to fulfill such a conditiondescribed above. It is to be noted that the high concentration region ofmetal oxide in the composite layer corresponds to a region having thehighest concentration of metal oxide in the first region, and the lowconcentration region corresponds to a region having the lowestconcentration of metal oxide in the second region.

Further, effect of film thicknesses of the first region and the secondregion on the composite layer is examined. A composite layer is formedunder a condition where the number of rotations of the first rotatingplate 502 is increased to 8 rpm compared to the rotation number in thecase of manufacturing the composite layer shown in FIG. 11, and otherconditions as the same with the above, where an evaporation rate of theorganic compound is 0.4 nm/s; and a weight ration of molybdenum oxideand DNTPD is 0.67:1. A cross section of the obtained object 505 a, inother words, a cross section of the composite layer is observed with theuse of a transmission electron microscope (TEM). Further, the obtainedTEM photograph is shown in FIG. 12.

In FIG. 12, it can also be found that a first region with a dark colorand a second region with a light color are alternately provided,similarly to in FIG. 11. As described above, the first region with adark color is a region having a high concentration of molybdenum oxide,and the second region with a light color is a region having a highconcentration of DNTPD.

However, it is found from FIG. 12 that one cycle of a periodic change ofthe first region and the second region, in other words, one cycle of aperiodic change of a concentration of molybdenum oxide is about 3 nm inthe composite layer manufactured by increasing the number of rotationsof the first rotating plate 502 in the stacked direction, while the onecycle of a periodic change is about 12 nm in FIG. 11.

Characteristics of the composite layers shown in FIG. 11 and FIG. 12 areexamined with the use of an element below. The element used for theexamination has a structure in which the composite layer shown in FIG.11 or FIG. 12 is formed to have a thickness of 120 nm after forming ITOcontaining silicon oxide over the substrate, and aluminum is formed tohave a thickness of 300 nm thereover.

It is to be noted that an element having the composite layer (FIG. 11)indicates an element 1, which is obtained in the case where the numberof rotations of the first rotating plate 502 is set to be 2 rpm, and thecomposite layer (FIG. 12) indicates an element 2, which is obtained inthe case where the number of rotations of the first rotating plate 502is set to be 8 rpm.

FIG. 13 shows current density-voltage characteristics of the element 1and the element 2. From FIG. 13, it is found that a currentcharacteristic of the element 2 is superior to a current characteristicof the element 1. Therefore, it is preferable that one cycle of periodicchange of a concentration of molybdenum oxide, in other words, aconcentration of metal oxide in the composite layer be further shorter.In accordance with the above, one cycle of periodic change of aconcentration of metal oxide is preferably 12 nm or less, furtherpreferably, 3 nm or less. Furthermore, a composite layer having auniform concentration of metal oxide in which one cycle of periodicchange is close to 0 nm is preferable. A rotation speed, an evaporationrate, a distance between an object and the evaporation source, adistance of the evaporation sources, a distance between the rotatingplate and the axis, and the like are appropriately designed to form acomposite layer so as to fulfill such a condition described above.

Further, a plurality of evaporation sources of metal oxide 601 a may beset as shown in FIG. 6. Though an example of five evaporation sources isshown in FIG. 6, two, three, four, or six or more of evaporation sourcesmay be provided. In a case of using such an evaporation apparatus,amount of evaporation can be easily controlled by opening and closingshutters provided over each evaporation source. Therefore, a compositelayer, which has a stacked-layer of a composite layer having differentconcentrations of metal oxide and a concentration gradient, can beeasily manufactured without decreasing a temperature of an evaporationsource.

Evaporation rates of metal oxide and an organic compound may be the sameor different between each material, and it is appropriately selecteddepending on a concentration of metal oxide that is to be formed.Further, shapes of the first rotating plate 502 and the second rotatingplates 504 a to 504 d are not particularly limited. In addition to acircular form as shown in FIG. 5 and FIG. 6, a polygon such as aquadrangle may be employed. By providing the second rotating plates 504a to 504 d, inside variation of a thickness of a layer or the likeformed in an object can be reduced. It is to be noted that the secondrotating plates 504 a to 504 d are not always needed to be provided.

A structure inside the treatment camber 401 is not limited to thestructures shown in FIG. 5 and FIG. 6. For example, a structure in whichevaporation sources are shifted as shown in FIG. 7 may be employed.

In FIG. 7, a rotating plate 706 rotated around an axis 707 and to whichevaporation sources 701 a and 701 b are fixed, and a holder 702 forholding objects 705 a to 705 d are provided as being opposed to eachother. Metal oxide and an organic compound are respectively held in theevaporation source 701 a and the evaporation source 701 b. When theevaporation source 701 a is set to be closer to the object 705 a thanthe evaporation source 701 b is, co-evaporation is performed so that aconcentration of metal oxide is higher than a concentration of anorganic compound over the object 705 a. When the evaporation source 701b is set to be closer to the object 705 a than the evaporation source701 a is by rotating the rotating plate 706, co-evaporation is performedso that a concentration of an organic compound is higher than aconcentration of metal oxide over the object 705 a. In such a manner, anevaporation apparatus may have a structure in which the position of theevaporation source with respect to the object is shifted by shifting theposition of the evaporation source. That is, the evaporation source andthe object may be provided so that each position is relatively sifted.

For the evaporation source, there are a resistance heating method thatperforms direct heating and a radiation heating method that performsindirect heating; however, both methods can be used for manufacturing acomposite layer. Any kinds of evaporation containers can be used. Forexample, a crucible 800 shown in FIG. 8A is an evaporation containerthat is usually used in a case of mass-production. An evaporationtemperature of molybdenum oxide that is used as metal oxide of thecomposite layer depends on a shape and a size of the crucible. However,since the evaporation temperature of molybdenum oxide is about 450 to550° C. in vacuums, it is a high evaporation temperature compared to anorganic compound. Therefore, a material of the evaporation container isnecessary to be considered. For the evaporation container, a non-metalmaterial such as aluminum nitride, boron nitride, silicon carbide, orboron phosphate is preferable. Alternatively, a composite material ofthese materials may be used. In addition, an arbitrary material such astantalum, tungsten, or alumina may be appropriately selected inconsideration of a working temperature, reactivity, or the like. Athickness of the crucible may be determined by considering expectedcontent and a shape of the evaporation material or thermal conductivityof the material, or the like.

Further, a crucible with a cap 801, which has an opening portion asshown in FIG. 8B may be used. As similar to the crucible 800, a materialof the cap 801 may be appropriately selected from one or a plurality ofarbitrary materials of aluminum nitride, boron nitride, silicon carbide,boron phosphate, tantalum, tungsten, alumina, or the like inconsideration of a working temperature, reactivity, or the like.

As a consideration point in a case of performing evaporation of metaloxide, the opening portion tends to be clogged up with the material.This is because a temperature of an opening portion in an upper side ofthe evaporation container is lower compared to a temperature in a lowerside of the evaporation container. In particular, the opening portion iseasily clogged up in a case where an evaporation rate of metal oxide ishigh.

In order to soak an inside of the crucible, means are preferablyimplemented as the following: number of rolling heaters in the upperside of the crucible is increased; a side surface in the upper side ofthe crucible is coated with a substance having high thermal conductivitysuch as silver, gold, copper, aluminum; or a particle 803 havingfavorable thermal conductivity such as boron nitride (thermalconductivity: 60 W·m⁻¹·K⁻¹), silicon carbide (thermal conductivity: 270W·m⁻¹·K⁻¹), aluminum nitride (thermal conductivity: 70 W·m⁻¹·K⁻¹ or moreand 320 W·m⁻¹·K⁻¹ or less), or boron phosphate is put into the crucible800 with metal oxide 802 that is an evaporation material as shown inFIG. 8C; or the like. Further, these means can be appropriately combinedwith each other and it is more effective to soaking. Mixture of theparticle 803 and the evaporation material has effect that bumping of theevaporation material is prevented. A grain size of the particle 803 ispreferably 0.1 mm or more and 5 mm or less in a diameter. For a shape ofthe particle 803, the particle with a spherical shape is shown here;however, a shape of the particle 803 is not particularly limited. Theparticle with an ovoid shape, an oval spherical shape like a go stone,an oval spherical shape like a rugby ball, a disc shape, a cylindricalshape, or a polygonal prism shape may be used.

The crucible filled with metal oxide is described here; however, thesimilar thing to the above is applied to a crucible filled with anorganic compound.

It is to be noted that the present embodiment mode can be appropriatelycombined with Embodiment Mode 1 or Embodiment Mode 2.

(Embodiment Mode 4)

In the present embodiment mode, a light emitting device having a lightemitting element of the present invention will be explained withreference to FIGS. 9A and 9B. FIG. 9A shows a top view showing a lightemitting device, and FIG. 9B shows a cross-sectional view of A-A′ linein FIG. 9A (a cross-sectional view taken along A-A′). Reference numeral900 denotes a substrate. Reference numeral 901 shown by a dot linedenotes a driver circuit portion (a source side driver circuit).Reference numeral 902 denotes a pixel portion. Reference numeral 903denotes a driver circuit portion (a gate side driver circuit). Further,reference numeral 904 denotes a sealing substrate, and reference numeral905 denotes a sealing material. The inside surrounded by the sealingmaterial 905 is a space 906.

Reference numeral 907 denotes a wiring for transmitting signals input tothe source side driver circuit 901 and the gate side driver circuit 903,which receives signals such as a video signal, a clock signal, a startsignal, and a reset signal from a FPC (Flexible Printed Circuit) 908that is an external input terminal. Although only the FPC is shown here,a printed wiring board (PWB) may be attached to the FPC. The category ofthe light emitting device of the present invention includes not onlylight emitting devices themselves but also light emitting devices towhich an FPC or a PWB is attached.

Next, the sectional structure will be explained with reference to FIG.9B. The driver circuit portion and the pixel portion are formed over thesubstrate 900. Here, the source side driver circuit 901 that is one ofthe driver circuit portions and the pixel portion 902 are shown.

In the source side driver circuit 901, a CMOS circuit, in which ann-channel TFT 923 and a p-channel TFT 924 are combined, is formed. Thedriver circuit constituted by TFTs may be formed with a known CMOScircuit, PMOS circuit, or NMOS circuit. Although the present embodimentmode shows the case that driver circuits are formed over the samesubstrate, the driver circuits are not necessarily formed over the samesubstrate, and the driver circuits can be formed outside the substrate.

The pixel portion 902 is constituted by a plurality of pixels, each ofwhich includes a switching TFT 911, a current controlling TFT 912, and afirst electrode 913 electrically connected to a drain of the currentcontrolling TFT 912. An insulator 914 is formed to cover an end portionof the first electrode 913. Here, a positive photosensitive acrylicresin film is used to form the insulator 914.

In addition, an upper or lower end portion of the insulator 914 ispreferably made to have a curved surface with a curvature in order toform a desirable layer including a light-emitting substance 916 to beformed later. For example, in a case of using positive photosensitiveacrylic as a material of the insulator 914, it is preferable that onlythe upper end portion of the insulator 914 be made to have a curvedsurface with a curvature radius (0.2 μm to 3 μm). Besides, it ispossible to use a negative photosensitive material that is insoluble inan etchant by photosensitive light or a positive photosensitive materialthat is soluble in an etchant by photosensitive light as the insulator914. Further, not only organic materials but also inorganic materialscan be used as the material of the insulator 914, and for example,silicon oxide, silicon oxynitride or the like can be used.

The layer including a light-emitting substance 916 and a secondelectrode 917 are formed over the first electrode 913 by the each methodshown in the above embodiment modes. When at least a light emittinglayer and a composite layer between the light emitting layer and one ofthe electrodes are provided in the layer including a light-emittingsubstance 916, other layers are not particularly limited, and any oflayers can be appropriately selected.

The sealing substrate 904 and the substrate 900 are bonded to each otherwith the sealing material 905, and thus, a structure can be obtained, inwhich a light emitting element 918 is provided in the space 906surrounded by the substrate 900, the sealing substrate 904, and thesealing material 905. The light emitting element 918 includes the firstelectrode 913, the layer including a light-emitting substance 916, andthe second electrode 917. There is a case that the space 906 is filledwith the sealing material 905, in addition to a case that the space 906is filled with an inert gas (such as nitrogen or argon).

It is to be noted that it is preferable to use an epoxy resin for thesealing material 905. Such a material that hardly transmits moisture andoxygen is preferable. Further, as a material used for the sealingsubstrate 904, a plastic substrate made of FRP (Fiberglass-ReinforcedPlastics), PVF (polyvinyl fluoride), Mylar, polyester, acrylic, or thelike can be used as well as a glass substrate or a quartz substrate.

As described above, a light emitting device manufactured by using thepresent invention can be obtained. The light emitting device hassuperiority in a hole transporting property because it has a compositelayer. Therefore, a driving voltage can be reduced. Further, byselecting a concentration of metal oxide in the composite layer inconsideration of a refraction index, reflectivity of light in filminterfaces through a light emitting layer, the composite layer, andelectrodes can be reduced so that light extraction efficiency can beimproved. Accordingly, a light emitting device with low powerconsumption can be obtained.

The present embodiment mode can be appropriately combined with anystructures of Embodiment Modes 1 to 3.

The present invention is not limited to the above embodiment mode.

(Embodiment Mode 5)

In the present embodiment mode, various electronic apparatuses, whichare completed by using a light emitting device having a light emittingelement of the present invention, will be explained. By applying thepresent invention, a light emitting element with a low driving voltagecan be provided; therefore, an electronic apparatus equipped with alight emitting element of the present invention can achieve low powerconsumption.

As electronic apparatuses manufactured by using a light emitting deviceof the present invention, TV sets, cameras such as video cameras ordigital cameras, goggle-type displays (head mounted displays),navigation systems, sound reproduction devices (such as car audios oraudio components), personal computers such as laptop computers, gamemachines, portable information terminals (such as mobile computers, cellphones, portable game machines, or electronic books), image reproductiondevices provided with a recording medium (specifically, devices that canreproduce a recording medium such as a digital versatile disk (DVD) andare equipped with a display device capable of displaying the image) andthe like can be given. Some specific examples of electronic apparatusesare explained with reference to FIGS. 10A to 10D. Electronic apparatusesusing a light emitting device of the present invention is not limited tothese specific examples shown here.

FIG. 10A shows a display device, which includes a housing 1000, asupporting stand 1001, a display portion 1002, speaker portions 1003, avideo input terminal 1004, and the like. The display device ismanufactured by using a light emitting device of the present inventionfor the display portion 1002. It is to be noted that the category of thedisplay device includes all types of information display devices, forexample, display devices for a personal computer, display devices for TVbroadcast reception, display devices for advertisement display, and thelike.

A light emitting element of the present invention is provided in thedisplay portion 1002. The light emitting element of the presentinvention has superiority in a hole transporting property because it hasa composite layer including metal oxide and an organic compound betweena first electrode and a light emitting layer. Therefore, a drivingvoltage can be reduced. Further, by selecting a concentration of metaloxide in the composite layer in consideration of a refraction index,reflectivity of light in film interfaces through a light emitting layerto the first electrode can be reduced so that light extractionefficiency can be improved. Accordingly, a display device with low powerconsumption can be obtained.

FIG. 10B shows a laptop personal computer, which includes a main body1010, a housing 1011, a display portion 1012, a keyboard 1013, anexternal connection port 1014, a pointing mouse 1015, and the like.

A light emitting element of the present invention is provided in thedisplay portion 1012. The light emitting element of the presentinvention has superiority in a hole transporting property because it hasa composite layer including metal oxide and an organic compound betweena first electrode and a light emitting layer. Therefore, a drivingvoltage can be reduced. Further, by selecting a concentration of metaloxide in the composite layer in consideration of a refraction index,reflectivity of light in film interfaces through a light emitting layerto the first electrode can be reduced so that light extractionefficiency can be improved. Accordingly, a personal computer with lowpower consumption can be obtained.

FIG. 10C shows a video camera, which includes a main body 1020, adisplay portion 1021, a housing 1022, an external connection port 1023,a remote control receiving portion 1024, an image receiving portion1025, a battery 1026, an audio input portion 1027, operation keys 1028,an eyepiece portion 1029, and the like.

A light emitting element of the present invention is provided in thedisplay portion 1021. The light emitting element of the presentinvention has superiority in a hole transporting property because it hasa composite layer including metal oxide and an organic compound betweena first electrode and a light emitting layer. Therefore, a drivingvoltage can be reduced. Further, by selecting a concentration of metaloxide in the composite layer in consideration of a refraction index,reflectivity of light in film interfaces through a light emitting layerto the first electrode can be reduced so that light extractionefficiency can be improved. Accordingly, a video camera with low powerconsumption can be obtained.

FIG. 10D shows a cell phone, which includes a main body 1030, a housing1031, a display portion 1032, an audio input portion 1033, an audiooutput portion 1034, operation keys 1035, an external connection port1036, an antenna 1037, and the like.

A light emitting element of the present invention is provided in thedisplay portion 1032. The light emitting element of the presentinvention has superiority in a hole transporting property because it hasa composite layer including metal oxide and an organic compound betweena first electrode and a light emitting layer. Therefore, a drivingvoltage can be reduced. Further, by selecting a concentration of metaloxide in the composite layer in consideration of a refraction index,reflectivity of light in film interfaces through a light emitting layerto the first electrode can be reduced so that light extractionefficiency can be improved. Accordingly, a cell phone with low powerconsumption can be obtained.

As described above, an application range of the present invention isextremely wide, and the present invention can be used in display devicesin any fields. Further, the electronic apparatuses of the presentembodiment mode can be appropriately combined with any structures ofEmbodiment Modes 1 to 4.

This application is based on Japanese Patent Application serial no.2005-191401 filed in Japan Patent Office on June 30 in 2005, the entirecontents of which are hereby incorporated by reference.

1. A light emitting device comprising: a first electrode; a firstcomposite layer over the first electrode; a second composite layer overthe first composite layer; and a second electrode over the secondcomposite layer, wherein the first composite layer comprises a firstmetal, oxygen, and carbon, and the second composite layer comprises asecond metal, oxygen, and carbon, and wherein an average concentrationof the first metal in the first composite layer is higher than anaverage concentration of the second metal in the second composite layer.2. The light emitting device according to claim 1, wherein the firstmetal and the second metal are derived from a first metal oxide and asecond metal oxide, respectively.
 3. The light emitting device accordingto claim 2, wherein each of the first metal oxide and the second metaloxide is one or plural kinds of titanium oxide, vanadium oxide, chromiumoxide, zirconium oxide, niobium oxide, molybdenum oxide, hafnium oxide,tantalum oxide, tungsten oxide, and rhenium oxide.
 4. An electronicapparatus comprising the light emitting device according to claim
 1. 5.The light emitting device according to claim 1, further comprising: athird composite layer over the second composite layer, and between thesecond composite layer and the second electrode, wherein the thirdcomposite layer comprises a third metal, oxygen, and carbon.
 6. Thelight emitting device according to claim 5, wherein the averageconcentration of the second metal in the second composite layer ishigher than an average concentration of the third metal in the thirdcomposite layer.
 7. The light emitting device according to claim 5,wherein the first metal, the second metal, and the third metal arederived from a first metal oxide, a second metal oxide, and a thirdmetal oxide, respectively.
 8. The light emitting device according toclaim 7, wherein each of the first metal oxide, the second metal oxide,and the third metal oxide is one or plural kinds of titanium oxide,vanadium oxide, chromium oxide, zirconium oxide, niobium oxide,molybdenum oxide, hafnium oxide, tantalum oxide, tungsten oxide, andrhenium oxide.
 9. An electronic apparatus comprising the light emittingdevice according to claim
 5. 10. A light emitting device comprising: afirst electrode; a first composite layer over the first electrode; asecond composite layer over the first composite layer; and a secondelectrode over the second composite layer, wherein the first compositelayer comprises a first metal, oxygen, and carbon, and the secondcomposite layer comprises a second metal, oxygen, and carbon, wherein anaverage concentration of the first metal in the first composite layer ishigher than an average concentration of the second metal in the secondcomposite layer, and wherein at least one of the first composite layerand the second composite layer comprises an alternating stack oflaminations of a first region and a second region, the first regioncontaining a larger amount of the first or second metal and the secondregion containing a larger amount of the carbon.
 11. The light emittingdevice according to claim 10, wherein a concentration of the first metalin the first region is equal to or higher than a concentration of thefirst metal in the second region in the first composite layer.
 12. Thelight emitting device according to claim 10, wherein a highestconcentration of the first metal in the first region is the same or atmost eight times as a lowest concentration of the first metal in thesecond region in the first composite layer.
 13. The light emittingdevice according to claim 10, wherein a concentration of the secondmetal in the first region is equal to or higher than a concentration ofthe second metal in the second region in the second composite layer. 14.The light emitting device according to claim 10, wherein a highestconcentration of the second metal in the first region is the same or atmost eight times as a lowest concentration of the second metal in thesecond region in the second composite layer.
 15. The light emittingdevice according to claim 10, wherein the first metal and the secondmetal are derived from a first metal oxide and a second metal oxide,respectively.
 16. The light emitting device according to claim 15,wherein each of the first metal oxide and the second metal oxide is oneor plural kinds of titanium oxide, vanadium oxide, chromium oxide,zirconium oxide, niobium oxide, molybdenum oxide, hafnium oxide,tantalum oxide, tungsten oxide, and rhenium oxide.
 17. An electronicapparatus comprising the light emitting device according to claim 10.18. The light emitting device according to claim 10, further comprising:a third composite layer over the second composite layer, and between thesecond composite layer and the second electrode, wherein the thirdcomposite layer comprises a third metal, oxygen, and carbon.
 19. Thelight emitting device according to claim 18, wherein the averageconcentration of the second metal in the second composite layer ishigher than an average concentration of the third metal in the thirdcomposite layer.
 20. The light emitting device according to claim 18,wherein a concentration of the first metal in the first region is equalto or higher than a concentration of the first metal in the secondregion in the first composite layer.
 21. The light emitting deviceaccording to claim 18, wherein a highest concentration of the firstmetal in the first region is the same or at most eight times as a lowestconcentration of the first metal in the second region in the firstcomposite layer.
 22. The light emitting device according to claim 18,wherein a concentration of the second metal in the first region is equalto or higher than a concentration of the second metal in the secondregion in the second composite layer.
 23. The light emitting deviceaccording to claim 18, wherein a highest concentration of the secondmetal in the first region is the same or at most eight times as a lowestconcentration of the second metal in the second region in the secondcomposite layer.
 24. The light emitting device according to claim 18,wherein a concentration of the third metal in the first region is equalto or higher than a concentration of the third metal in the secondregion in the third composite layer.
 25. The light emitting deviceaccording to claim 18, wherein a highest concentration of the thirdmetal in the first region is the same or at most eight times as a lowestconcentration of the third metal in the second region in the thirdcomposite layer.
 26. The light emitting device according to claim 18,wherein the first metal, the second metal, and the third metal arederived from a first metal oxide, a second metal oxide, and a thirdmetal oxide, respectively.
 27. The light emitting device according toclaim 26, wherein each of the first metal oxide, the second metal oxide,and the third metal oxide is one or plural kinds of titanium oxide,vanadium oxide, chromium oxide, zirconium oxide, niobium oxide,molybdenum oxide, hafnium oxide, tantalum oxide, tungsten oxide, andrhenium oxide.
 28. An electronic apparatus comprising the light emittingdevice according to claim
 18. 29. A light emitting device comprising: afirst electrode; a first composite layer over the first electrode; asecond composite layer over the first composite layer; and a secondelectrode over the second composite layer, wherein the first compositelayer comprises a first metal, oxygen, and carbon, and the secondcomposite layer comprises a second metal, oxygen, and carbon, wherein anaverage concentration of the first metal in the first composite layer ishigher than an average concentration of the second metal in the secondcomposite layer, wherein at least one of the first composite layer andthe second composite layer comprises an alternating stack of laminationsof a first and a second region, the first region containing a largeramount of the first or second metal and the second region containing alarger amount of the carbon, and wherein each concentration of the firstmetal in the first composite layer and the second metal in the secondcomposite layer changes periodically in the stacked direction.
 30. Thelight emitting device according to claim 29, wherein a concentration ofthe first metal in the first region is equal to or higher than aconcentration of the first metal in the second region in the firstcomposite layer.
 31. The light emitting device according to claim 29,wherein a highest concentration of the first metal in the first regionis the same or at most eight times as a lowest concentration of thefirst metal in the second region in the first composite layer.
 32. Thelight emitting device according to claim 29, wherein a concentration ofthe second metal in the first region is equal to or higher than aconcentration of the second metal in the second region in the secondcomposite layer.
 33. The light emitting device according to claim 29,wherein a highest concentration of the second metal in the first regionis the same or at most eight times as a lowest concentration of thesecond metal in the second region in the second composite layer.
 34. Thelight emitting device according to claim 29, wherein the first metal andthe second metal are derived from a first metal oxide and a second metaloxide, respectively.
 35. The light emitting device according to claim34, wherein each of the first metal oxide and the second metal oxide isone or plural kinds of titanium oxide, vanadium oxide, chromium oxide,zirconium oxide, niobium oxide, molybdenum oxide, hafnium oxide,tantalum oxide, tungsten oxide, and rhenium oxide.
 36. The lightemitting device according to claim 29, wherein one cycle of a periodicchange is 12 nm or less.
 37. An electronic apparatus comprising thelight emitting device according to claim
 29. 38. The light emittingdevice according to claim 29, further comprising: a third compositelayer over the second composite layer, and between the second compositelayer and the second electrode, wherein the third composite layercomprises a third metal, oxygen, and carbon, and wherein a concentrationof the third metal changes periodically in the stacked direction. 39.The light emitting device according to claim 38, wherein the averageconcentration of the second metal in the second composite layer ishigher than an average concentration of the third metal in the thirdcomposite layer.
 40. The light emitting device according to claim 38,wherein a concentration of the first metal in the first region is equalto or higher than a concentration of the first metal in the secondregion in the first composite layer.
 41. The light emitting deviceaccording to claim 38, wherein a highest concentration of the firstmetal in the first region is the same or at most eight times as a lowestconcentration of the first metal in the second region in the firstcomposite layer.
 42. The light emitting device according to claim 38,wherein a concentration of the second metal in the first region is equalto or higher than a concentration of the second metal in the secondregion in the second composite layer.
 43. The light emitting deviceaccording to claim 38, wherein a highest concentration of the secondmetal in the first region is the same or at most eight times as a lowestconcentration of the second metal in the second region in the secondcomposite layer.
 44. The light emitting device according to claim 38,wherein a concentration of the third metal in the first region is equalto or higher than a concentration of the third metal in the secondregion in the third composite layer.
 45. The light emitting deviceaccording to claim 38, wherein a highest concentration of the thirdmetal in the first region is the same or at most eight times as a lowestconcentration of the third metal in the second region in the thirdcomposite layer.
 46. The light emitting device according to claim 38,wherein the first metal, the second metal, and the third metal arederived from a first metal oxide, a second metal oxide, and a thirdmetal oxide, respectively.
 47. The light emitting device according toclaim 46, wherein each of the first metal oxide, the second metal oxide,and the third metal oxide is one or plural kinds of titanium oxide,vanadium oxide, chromium oxide, zirconium oxide, niobium oxide,molybdenum oxide, hafnium oxide, tantalum oxide, tungsten oxide, andrhenium oxide.
 48. The light emitting device according to claim 38,wherein one cycle of a periodic change is 12 nm or less.
 49. Anelectronic apparatus comprising the light emitting device according toclaim 38.